Help with Spanning tree loop guard blocking

[Ollie Colebrook]

Hi everyone,

Looking for some help with one of our 3850 stacks which is getting this within its logs:

SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port Port-channel1 on VLAN0102.

The stack has 1 uplink to our core switch configured with the below configuration:

interface TenGigabitEthernet2/1/3
description LAG to Core
switchport mode trunk
channel-group 1 mode active

PortChannel 1 config:

interface Port-channel1
description LAG to core
switchport mode trunk

This issue seems to have only started within the last 24hrs, as interface Po1 is the uplink to the core when it is blocking it is affecting users.

Any help would be appreciated greatly !

[Giuseppe Larosa]

Hello @Ollie Colebrook ,

you have probably enabled loop guard at global level as we don't see spanning-tree commands in the port-channel configuration.

In my understanding and experience the error is triggered by the fact that in the affected VLAN 102 the access switch does not receive BPDUs coming from the root bridge on the uplink port. 3 missed  in a row are enough for Rapid STP

Instead of promoting itself to root bridge for VLAN 102 the switch for the loop guard feature moves the port in an inconsistent state for Vlan 102 waiting to receive again STP BPDUs from the core / root bridge.

when this happens the port is moved again to normal state.

Users of Vlan 102 are impacted.

Look for input errors and traffic level on the member links and verify that the root bridge for VLAN 102 is the expected one.

Hope to help

Giuseppe

[Ollie Colebrook]

Hi @Giuseppe Larosa Thank you for the information.

Yes correct it looks like its enabled at the global level.

Using the commands show spanning-tree inconsistent ports it shows 0 ?

Thank you

[paul driver]

Hello

You shouldnt really lose connection when you have a PC even with loopguard intiating as only the affected physical link should become blocked,  So can you confirm if you do only have a one physical link in the PC ?

[Ollie Colebrook]

Hi @paul driver 

Correct there is only 1 physical interface connected to this switch stack from our core switch. There should be two interfaces but it looks like this was missed. If we were to have two interfaces to the core both within the PC would this help the issue ?

Regards,
Ollie